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(11) | EP 1 783 095 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Excitation in micromechanical devices |
| (57) A resonant structure for a micromechanical device. The resonant structure comprises
a beam and at least one mass attached to the beam. The resonant structure is arranged
to have a predominantly rotational excitation mode and an excitation plane in which
motion of the excited resonant structure predominantly takes place in use, the at
least one mass having a geometry such that none of the principal axes of the rotational
inertia tensor of the resonant structure are normal to the excitation plane.
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[0002] Micromechanical sensing devices are known which sense external movement through changes in the motion of an excited sensing element. An example of such a sensing device is an angular rate sensor, used in applications such as rollover detection for vehicles.
[0003] The necessary positioning of components in a device such as an angular rate sensor is such that the excitation of the resonant structure which comprises the sensing element has to be driven in a plane normal to the desired plane of the excited motion. In an angular rate sensor, excitation electrodes must excite the sensing element of the sensor to a vibration mode which is essentially a motion parallel to the surface of the excitation electrodes. This means that the device must be designed so that the excitation motion has components in a direction normal to the desired plane of the excited motion.
[0004] A sensing element of such a sensor comprises one or more masses attached to a beam. Current devices achieve the desired excitation characteristics by using beams with a specially designed geometry. The cross-section of the beam is created to be asymmetric and have a geometry such that its principal axis is not parallel to the surface normal of the mass (see figure 1). This means that the beam has a tendency to bend out of the surface. The result is that an excitation mode has small components out of the surface plane of the mass and hence can be driven electrostatically from the side of the plane of the mass.
[0005] A problem with this approach is that, due to the relatively small sizes of the beams, process tolerances influence the properties and principal axis of the beam, Process tolerance control is therefore a major issue in the fabrication process. This is also the case for other micromechanical devices using resonant structures. The present invention aims to overcome this problem.
[0006] According to the present invention there is provided a resonant structure for a micromechanical device, the resonant structure comprising:
a beam;
at least one mass attached to the beam;
wherein the resonant structure is arranged to have a predominantly rotational excitation mode and an excitation plane in which motion of the excited resonant structure predominantly takes place in use, the at least one mass having a geometry such that none of the principal axes of the rotational inertia tensor of the resonant structure are normal to the excitation plane.[0007] According to the present invention there is further provided a micromechanical device comprising:
at least one resonant structure as defined above;
excitation means arranged to excite, in use, motion of the resonant structure in use.
[0008] According to the present invention there is further provided a method of exciting a resonant structure in an excitation plane in a micromechanical device, the method comprising the steps of:
providing a resonant structure comprising at least one mass having a geometry such that none of the principal axes of the rotational inertia tensor of the resonant structure are normal to the excitation plane; and
exciting motion of the resonant structure in a direction normal to the excitation plane.
[0009] Examples of the present invention will now be described with reference to the accompanying drawings, in which:
Figure 1 is a schematic cross-sectional diagram of a prior art sensing element;
Figure 2 is a schematic cross-sectional diagram of the concept of a sensing element according to the present invention;
Figures 3A to 3E show perspective diagrams of sensing elements according to the present invention with various example geometries; and
Figure 4 shows an example micromechnical device comprising a sensing element, according to the present invention.
[0010] Referring to figure 2, a resonant structure according to the invention comprises a beam 1 with a regularly shaped, for example rectangular, cross-section and one or more masses 2 with irregularly shaped cross-sections. The resonant structure may be, for example, a sensing element or part of a device which 'harvests' energy from rotation of a wheel for powering a device such as a sensing device. This is compared to prior art sensing elements, depicted in figure 1, which comprise a beam 3 with an irregular cross-section and regularly shaped masses 4.
[0011] Examples of different possible geometries of the masses 2 are shown in figure 3A to 3E, with altered principal axes shown as x' and z'. The geometry of the masses 2 is such that their rotational inertial mass tensor has principal axes that are not normal to the surface of the masses (xy -plane in figures). This results in an excitation mode that has (small) components out of the surface, which enables excitation and detection of the resonant structure to occur from a single side of the resonant structure, which may be a sensing element.
[0012] In figures 3A to 3E , it would be desired to excite motion in the xy-plane. Then, for example, external angular movement (such as of a vehicle in which the sensing element is located) causing Coriolis forces on the resonant structure can be detected by detecting the resultant movement of the resonant structure in the z-direction. The existence of components of the excited mode normal to the xy-plane, i.e. in the z-direction, means that it is possible to indirectly drive the desired excited motion in the xy-plane by use of excitation means which act to cause excitation in the z-direction.
[0013] An example micromechanical sensing device comprising a resonant structure functioning as a sensing element of the present invention is shown in figure 4. This example device has excitation electrodes 5 and detection electrodes 6 and senses angular rate about an axis parallel to the xy-plane. A device may comprise more than one such resonant structure.
[0014] With a sensing element according to the present invention it is possible to make a sensing device which exhibits equivalent behaviour to conventional devices with irregular beam cross-sections, but with the advantage of the device being more robust towards process tolerances.
[0015] Another example use for a resonant structure according to the present invention is in a power generation device for use on a wheel, where Coriolis movement can be converted into useful energy, some of which can be fed back into the device to drive the excitation means and some of which can be used to power a sensing device on the wheel. The resonant structure is suitable for use in any such device where the vibratory motion is mainly rotational.
1. A resonant structure for a micromechanical device, the resonant structure comprising:
a beam;
at least one mass attached to the beam;
wherein the resonant structure is arranged to have a predominantly rotational excitation
mode and an excitation plane in which motion of the excited resonant structure predominantly
takes place in use, the at least one mass having a geometry such that none of the
principal axes of the rotational inertia tensor of the resonant structure are normal
to the excitation plane.
2. A resonant structure according to claim 1, wherein the beam has a rectangular cross-section
normal to the excitation plane.
3. A resonant structure according to claim 1 or claim 2, wherein the at least one mass
is asymmetric about an axis through the centre of the beam and parallel to the length
of the beam.
4. A resonant structure according to claim 1 or claim 2, wherein the at least one mass
has two-fold rotational symmetry about an axis through the centre of the beam and
parallel to the length of the beam but no mirror symmetry about the same axis.
5. A resonant structure according to any preceding claim, wherein the resonant structure
is a sensing element.
6. A micromechanical device comprising:
at least one resonant structure according to any preceding claim;
excitation means arranged to excite, in use, motion of the resonant structure in use.
7. A device according to claim 6, wherein the device is a sensing device and the excitation
means is arranged to excite, in use, motion of the sensing element in a direction
normal to the excitation plane;
the device further comprising detection means arranged to detect, in use, motion in a direction normal to the excitation plane not caused by the excitation means.
the device further comprising detection means arranged to detect, in use, motion in a direction normal to the excitation plane not caused by the excitation means.
8. A device according to claim 6 or claim 7, wherein the resonant structure is arranged
to be excitable to a predominantly rotational excitation mode.
9. A device according to claim 6, wherein the device is a power generation device and
the resonant structure is arranged to be excitable to a predominantly rotational excitation
mode, the resonant structure being arranged to be fixed to a wheel in use, such that
the rotation axis of the excited resonant structure is substantially perpendicular
to the rotation axis of the wheel;
the device further comprising power extraction means arranged to convert, in use, Coriolis motion of the excited resonant structure to generate power.
the device further comprising power extraction means arranged to convert, in use, Coriolis motion of the excited resonant structure to generate power.
10. A device according to any of claims 6 to 9, wherein the excitation means is arranged
to electrostatically excite motion of the resonant structure.
11. A method of exciting a resonant structure in an excitation plane in a micromechanical
device, the method comprising the steps of:
providing a resonant structure comprising at least one mass having a geometry such that none of the principal axes of the rotational inertia tensor of the resonant structure are normal to the excitation plane; and
exciting motion of the resonant structure in a direction normal to the excitation plane.